It is known that, in nuclear reactors, the cover is fastened removably to the vessel, to make it possible to recharge the reactor with fuel periodically and, as a result of this, inspect the interior of the vessel.
For this purpose, the cover is fastened to the reactor vessel by means of bolts which are screwed into the collar of the vessel and which penetrate into smooth holes provided in the collar of the cover. These bolts are usually used in fairly large numbers and are relatively close to ane another, with the result that, to remove and refit the cover, it is necessary to unscrew and screw all these bolts as well as to tension them.
These operations of screwing or unscrewing the bolts are problematical and difficult to carry out. In fact, the bolts weigh several hundred kilograms, so that, when the bolt is screwed into the thread or when it is unscrewed, it is necessary to overcome frictional resistances which occur in the thread and which are greater, the heavier the bolt. Moreover, the axes of the various elements of the screwing appliance, after operation, do not always coincide with the axes of the tapped holes in the vessel or with the axes of the bolts. Consequently, it often happens that the bolt cannot automatically come in line coaxially at the moment when it is introduced into the tapped hole in the vessel, and this operation therefore has to be carried out manually, as do the first turns to screw in the bolt. As a result, during this work the personnel risk staying for an excessively long period in a zone where the radioactive radiation is too high.
These defects in alignment, which can also cause the bolt to seize when it is screwed into the tapped hole in the vessel, set up in the moving parts irregular stresses which are distributed about their axis of translation or rotation and which can cause through friction irregular and premature wear of these parts.
A known appliance for screwing and unscrewing bolts allows a certain freedom of axial orientation of the bolt by means of a ball-and-socket and air-cushion system combined with a driving joint having two degrees of freedom in two directions perpendicular to one another.
However, this appliance is extremely complicated and does not make it possible to locate the preferred screwing direction during the fitting of the bolts, with the result that excessive friction can arise on the moving parts.